A technique for fabricating an improved electronic enclosure. The electronic enclosure is made of graphite fibers dispersed directionally (non-homogeneous) in an absolac/polycarbonate (ABS/PC) resin mix, which the composition is molded to form the plastic enclosure. The graphite concentration is highest along the interior surface of the enclosure to provide improved heat transfer, as well as adequate EMI/RFI shielding. However, the graphite concentration decrease along the thickness, wherein at the outer surface, the graphite concentration level is zero. The directional variation in the graphite loading allows high graphite loading at the interior surface of the enclosure, but retains lower loading at other regions along the thickness so that rigidity and impact resistance are retained for the enclosure.
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1. A method of forming an enclosure for housing an electronic component, said method comprising:
adding a resin and an additive material to a mold, said mold to form said enclosure; and varying an amount of said additive material during said adding to provide a varying concentration of said additive material along a thickness of said enclosure, said varying concentration being higher along an interior surface of said enclosure and lower along an exterior surface of said enclosure, said higher concentration of additive material being approximately 50% by volume.
3. The method of
4. The method of
5. The method of
6. The method of
7. The method of
providing said resin with a first injector; and injecting said additive with a second injector.
8. The method of
9. The method of
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This is a Divisional of application Ser. No. 09/090,709, filed Jun. 4, 1998 now U.S. Pat. No. 6,147,301.
1. Field of the Invention
The present invention relates to the field of enclosures and, more particularly, to electronic enclosures.
2. Background of the Related Art
The use of a plastic enclosure to house electronic components and assemblies is well known in the art. Most electronic enclosures, such as enclosures for notebook computers, are manufactured by an injection molding or compression molding technique. The plastic molding forms the outer shell or "skin" which provides the necessary structural rigidity, but is light in weight.
Several key requirements are specified when the enclosure is utilized for notebook computers. As noted above, the enclosure should be rigid, but light in weight, so that it can be hand-carried. The enclosure (or casing) should be resistant to cracking or breaking. For example, the enclosure should not shatter, if dropped. Further, the enclosure should be thermally conductive to dissipate heat, so that heat generated by internal components, such as power supplies, can be adequately transferred to the outer surface. Finally, some form of EMI/RFI shielding is needed to electrically shield the internal electronics.
One technique in practice utilizes an absolac/polycarbonate (ABS/PC) resin mix to fabricate the enclosures. Typically, a 60/40 mix of ABS/PC is employed in injection or compression molding to fabricate enclosures with thickness in the range of 1.5 to 2.0 millimeters (mm). A minimum thickness of about 1.5 mm is necessary for this thermoplastic material to provide adequate structural rigidity. The ABS/PC mix offers strength, impact resistance and is economically, cost effective.
However, several disadvantages are noted with the ABS/PC material. For example, the thermal conductivity of ABS/PC resin is quite low (typically less than 0.1 Watts per meter-Kelvin (W/m-K)), so that the heat spreading (dissipation) capability of the plastic is poor. Accordingly, many of today's notebook computers have "hot spots" along the external casing. Additionally, the ABS/PC plastic has poor electrical conductivity so that the interior surface of the enclosure requires some form of metallization (whether a metal skin or sprayed coating) for EMI/RFI shielding.
One technique to improve the properties of the ABS/PC resin material is to introduce graphite fibers into the resin. Graphite fibers are uniformly distributed in the resin when the enclosure is fabricated. Since graphite has higher thermal conductivity than ordinary ABS/PC, the graphite laden ABS/PC improves the thermal dissipation of the plastic. However, when significant amounts of graphite fibers are introduced to improve the thermal properties of the plastic, the amount of graphite present causes the graphite/ABS/PC resin based plastic to become brittle. This causes the impact resistance of the enclosure to degrade and increases the chances that the enclosure will shatter when dropped.
Accordingly, it would be advantageous to provide an enclosure having enhanced thermal conductivity, but without suffering the degradation of impact resistance. The present invention provides for such a scheme in which thermal conductivity is enhanced for a plastic enclosure, but in which the enclosure is not susceptible to breakage from impact, such as when the enclosure is dropped.
The present invention describes an improved electronic enclosure and the fabrication of such an enclosure. The electronic enclosure of the preferred embodiment is formed from a molded plastic, comprised of graphite fibers dispersed in an absolac/polycarbonate (ABS/PC) resin mix. The graphite loading in the formed enclosure is directional (non-homogeneous), so that the concentration of the graphite varies across the thickness of the formed plastic. The graphite concentration is highest along the interior surface of the enclosure and lowest (or none) along its outer surface. In the preferred graphite/ABS/PC enclosure, the graphite concentration proximal to the interior surface has a value of around 50% and a value of zero along the outer surface.
By having a high graphite concentration loading along the interior surface, thermal conductivity is improved to dissipate the heat away from the internal components. In many instances, hot spots along the outer surface are removed or reduced, due to the high thermal conductivity of the graphite along the interior surface. The improved thermal conductivity allows heat to conduct horizontally along the enclosure, as well as toward the exterior of the enclosure. Although high concentrations of graphite would cause the plastic to become brittle and the case to break (shatter), the enclosure of the present invention does not suffer from degraded impact performance. Since the high concentration levels of graphite are only along the interior surface, the structural rigidity of the ABS/PC based plastic is still retained for most of the thickness of the formed enclosure. Furthermore, the high concentration layer of graphite along the interior surface provides enhanced EMI/RFI shielding, so that a separate metallic skin along the interior of the enclosure is not necessary.
A technique for providing an improved electronic enclosure is described. In the following description, numerous specific details are set forth, such as specific materials, structures, processes, measurements, etc., in order to provide a thorough understanding of the present invention. However, it will be appreciated by one skilled in the art that the present invention may be practiced without these specific details. In other instances, well known techniques and structures have not been described in detail in order not to obscure the present invention.
Referring to
As noted in the Background section above, notebook computer enclosures are manufactured by an injection or compression molding technique. Therefore, the enclosure 11 (as well as cover 12 for most instances) is a thermoplastic molding that forms the outer shell or "skin" of the notebook computer. This shell needs to provide the necessary structural rigidity for holding the electronics, but it should be light in weight so that it can be hand-carried. The enclosure should be thermally conductive to dissipate heat and some form of electro-magnetic interference or radio frequency interference (EMI/RFI) shielding is needed to electrically shield the internal electronics. Metallization of the interior skin surface of the enclosure achieves this purpose.
One commonly practiced technique utilizes an absolac/polycarbonate (ABS/PC) resin mix to fabricate the enclosures. Typically, a 60/40 mix of ABS/PC is employed for injection or compression molding to fabricate enclosures with thickness in the range of 1.5 to 2.0 millimeters (mm). A minimum thickness of about 1.5 mm is necessary for this plastic to provide adequate structural rigidity. The ABS/PC mix offers strength, impact resistance and is economically, cost effective.
However, the thermal conductivity of ABS/PC resin is quite low (typically less than 0.1 Watts per meter-Kelvin (W/m-K)), so that the heat spreading (dissipation) capability of the plastic is poor. Accordingly, many of today's notebook computers have "hot spots" along the external surface of the casing. Additionally, the ABS/PC plastic has poor electrical conductivity so that the interior surface of the enclosure typically requires some form of metallization (whether a metal skin or sprayed coating) for EMI/RFI shielding.
Referring to
The hot spots which result from the poor thermal conductivity is illustrated in FIG. 4.
If the heat generation in the notebook computer is substantial and the enclosure cannot dissipate the heat adequately, some other mechanism (such as cross ventilation, or even a fan) will be needed to properly dissipate the heat from the interior of the enclosure 11 to the external environment. Additionally, with the enclosure 11 formed strictly from the ABS/PC resin, some form of metallic lining is required along the internal surface 21 for EMI/RFI shielding. Typically, metal plates or sprayed coating (skin) will be used to provide the metallization along the interior surface 21 of the enclosure 11.
One technique to improve the properties of the ABS/PC resin material is to introduce an additive into the resin to enhance thermal conductivity. One such additive is graphite. Graphite fibers, having thermal conductivity in the range 400 to 1100 W/m-K, are commercially available from various vendors (such as Amoco Corporation). The fibers are ground and mixed into the resin. Subsequently, the graphite/resin mixture is utilized in the molding process to form the plastic material.
For example, this technique is utilized to introduce graphite fibers into the ABS/PC resin to form a graphite/ABS/PC plastic, in which the graphite fiber composition (by volume) is approximately 40%. Since the graphite/resin composition is mixed into the 40%-60% ABS/PC ratio prior to molding, the resulting molded plastic has a homogeneous distribution of the graphite when the enclosure is fabricated. The cross-section of the formed graphite/ABS/PC plastic and the associated graphs are illustrated in
As noted earlier, the graphite has higher thermal conductivity than ordinary ABS/PC and, accordingly, the graphite/ABS/PC composition improves the thermal transfer across the material thickness Y. An added advantage of using graphite is its ability to function as a EMI/RFI shield. That is, with the presence of proper amounts of graphite material, it can provide the necessary EMI/RFI shielding without the need for a metallic lining along the inside surface of the enclosure.
It would appear that a significantly higher presence of graphite in the composition would further improve the thermal conductivity and shielding properties of the composite material. Unfortunately, when sufficiently high amounts of graphite fibers are introduced, the graphite causes the formed plastic to become brittle, which is an undesirable feature. The degradation in the impact resistance of the plastic material can result in the enclosure to shatter when dropped. Thus, a compromise ratio of graphite to the ABS/PC resin must be determined, which will provide adequate thermal and shielding properties, but will not cause the enclosure to become too brittle.
The example of
In order to solve the above problems, the present invention utilizes a directionally distributed graphite loading factor, which is illustrated in FIG. 6A.
The graph of
It is appreciated that the loading curve of
Several advantages are noted with the enclosure of the present invention over the prior art enclosures. A higher concentration of graphite loading can be used for electronic enclosures (such as enclosure 11). Since the high concentration levels are localized along one surface only, the low impact resistance properties of the material are also localized along this surface. However, since the remaining thickness of the material has lower concentration of graphite, the brittleness diminishes correspondingly, where upon at the outer surface, the impact resistance is at the highest value. With proper loading profiles, substantially high concentration levels of the graphite can be achieved at the interior surface 21, but the whole enclosure still has high impact resistance. The loading factor at the interior surface 21 can exceed loading factors which are not suitable for enclosures fabricated from uniformly loaded materials. Since the loading is substantially high along the interior surface, the EMI/RFI shielding properties are further enhanced. With sufficient graphite concentration, additional metallization is not needed along the interior surface.
Another advantageous feature is illustrated in FIG. 7.
It is appreciated that a variety of techniques can be employed to manufacture an enclosure having the directionally distributed graphite concentration, in which the graphite is loaded into a resin base, such as the afore-mentioned ABS/PC resin. For example, in one technique, the injection or compression molding technique known in the prior art is utilized. Furthermore, co-injection techniques allow for the injection of two different materials. Co-injection processes are provided by such vendors as Cincinnati Milacron of Cincinnati, Ohio and Co-Mack Technologies, Inc. of Vista, Calif.
Instead of using a single injector, two (or two sets of) injectors are used. The first injector feeds in the ABS/PC resin. The second injects the graphite/ABS/PC resin mixture having a concentration of graphite particles which correspond to the highest loading factor of graphite to be used. When forming the outer surface 22, only the ABS/PC injector is activated. When the graphite loading is to commence at a particular thickness, the second injector is activated. The amount of ABS/PC injection from the first injector and the amount of graphite/ABS/PC injection from the second injector are varied pursuant to the loading profile desired. When forming the interior region, the first injector is turned off, so that the layer adjacent to the interior is formed by the mixture from the second injector alone. It is appreciated that the injection loading profile can be computer controlled for accurately obtaining the desired graphite loading profile desired.
Accordingly, by the practice of the present invention, an improved electronic enclosure can be fabricated. The enclosure provides for enhanced heat transfer and dissipation, while maintaining structural rigidity which is not susceptible to breakage. The improved enclosure also provides EMI/RFI shielding without a separate metallic skin along the interior surface. In some instances, it may be possible to reduce the thickness of the enclosure wall, since the fiber-reinforced material can be structurally more rigid. Thus, a reduction in the size and/or weight of the portable device (such as the computer of
Thus, an improved electronic enclosure and a technique for fabricating such an enclosure is described.
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